秋田県谷地地域の初生的斜面崩壊に関する地質学的・岩石力学的背景  [in Japanese] Geological and Rock Mechanical Background of the Primary Slope Failure in Yachi Area, Akita Prefecture, Japan  [in Japanese]

The slope failure in Yachi area, the southeastern part of Akita prefecture, is derived from a typical primary landslide of bed rock in older age. This landslide along the left side of the river Narusegawa, of which course is controlled by geological structural line, isextensively developed in large scale with distance of about 1,300 m from east to west, about 950 m from south to north, and the volume of slidden mass amounts to about 35,000,000 m^3. The cause of this landslide can be essentially sought in the geological back ground, especially, in the lithology and geological struetures (the Naruse river fault and synclinorium structure) of the Tertiary strata forming almost this area. The geology in this area is mainly composed of three kinds of rock species that are different in each lithological characteristic, namely the so-called "hard shale", "intercalation of tuff" and "fault clay" that was formed by bedding slip. Based on physical and mechanical tests, the landslide surface is verified to have been formed along the fault clay and the intensely weathered tuff. The Yachi landslide at its primaly stage is of the slide type in UEMURA's criterion and of the rock block slide type in VARNAS's criterion.